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      Atrogin-1 Increases Smooth Muscle Contractility Through Myocardin Degradation : ATROGIN-1 AND SMOOTH MUSCLE CONTRACTILITY

      , , ,
      Journal of Cellular Physiology
      Wiley

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          Skeletal muscle hypertrophy and atrophy signaling pathways.

          Skeletal muscle hypertrophy is defined as an increase in muscle mass, which in the adult animal comes as a result of an increase in the size, as opposed to the number, of pre-existing skeletal muscle fibers. The protein growth factor insulin-like growth factor 1 (IGF-1) has been demonstrated to be sufficient to induce skeletal muscle hypertrophy. Over the past few years, signaling pathways which are activated by IGF-1, and which are responsible for regulating protein synthesis pathways, have been defined. More recently, it has been show that IGF-1 can also block the transcriptional upregulation of key mediators of skeletal muscle atrophy, the ubiquitin-ligases MuRF1 and MAFbx (also called Atrogin-1). Further, it has been demonstrated recently that activation of the NF-kappaB transcription pathway, activated by cachectic factors such as TNFalpha, is sufficient to induce skeletal muscle atrophy, and this atrophy occurs in part via NF-kappaB-mediated upregulation of MuRF1. Further work has demonstrated a trigger for MAFbx expression upon treatment with TNFalpha--the p38 MAPK pathway. This review will focus on the recent progress in the understanding of molecular signalling, which governs skeletal muscle atrophy and hypertrophy, and the known instances of cross-regulation between the two systems.
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            Degradation of MyoD mediated by the SCF (MAFbx) ubiquitin ligase.

            MyoD controls myoblast identity and differentiation and is required for myogenic stem cell function in adult skeletal muscle. MyoD is degraded by the ubiquitin-proteasome pathway mediated by different E3 ubiquitin ligases not identified as yet. Here we report that MyoD interacts with Atrogin-1/MAFbx (MAFbx), a striated muscle-specific E3 ubiquitin ligase dramatically up-regulated in atrophying muscle. A core LXXLL motif sequence in MyoD is necessary for binding to MAFbx. MAFbx associates with MyoD through an inverted LXXLL motif located in a series of helical leucine-charged residue-rich domains. Mutation in the LXXLL core motif represses ubiquitination and degradation of MyoD induced by MAFbx. Overexpression of MAFbx suppresses MyoD-induced differentiation and inhibits myotube formation. Finally the purified recombinant SCF(MAFbx) complex (SCF, Skp1, Cdc53/Cullin 1, F-box protein) mediated MyoD ubiquitination in vitro in a lysine-dependent pathway. Mutation of the lysine 133 in MyoD prevented its ubiquitination by the recombinant SCF(MAFbx) complex. These observations thus demonstrated that MAFbx functions in ubiquitinating MyoD via a sequence found in transcriptional coactivators. These transcriptional coactivators mediate the binding to liganded nuclear receptors. We also identified a novel protein-protein interaction module not yet identified in F-box proteins. MAFbx may play an important role in the course of muscle differentiation by determining the abundance of MyoD.
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              The FOXO3a transcription factor regulates cardiac myocyte size downstream of AKT signaling.

              Although signaling mechanisms inducing cardiac hypertrophy have been extensively studied, little is known about the mechanisms that reverse cardiac hypertrophy. Here, we describe the existence of a similar Akt/forkhead signaling axis in cardiac myocytes in vitro and in vivo, which is regulated by insulin, insulin-like growth factor (IGF), stretch, pressure overload, and angiotensin II stimulation. FOXO3a gene transfer prevented both IGF and stretch-induced hypertrophy in rat neonatal cardiac myocyte cultures in vitro. Transduction with FOXO3a also caused a significant reduction in cardiomyocyte size in mouse hearts in vivo. Akt/FOXO signaling regulated the expression of multiple atrophy-related genes "atrogenes," including the ubiquitin ligase atrogin-1 (MAFbx). In cardiac myocyte cultures, transduction with constitutively active Akt or treatment with IGF suppressed atrogin-1 mRNA expression, whereas transduction with FOXO3a stimulated its expression. FOXO3a transduction activated the atrogin-1 promoter in both cultured myocytes and mouse heart. Thus, in cardiomyocytes, as in skeletal muscle, FOXO3a activates an atrogene transcriptional program, which retards or prevents hypertrophy and is down-regulated by multiple physiological and pathological stimuli of myocyte growth.
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                Author and article information

                Journal
                Journal of Cellular Physiology
                J. Cell. Physiol.
                Wiley
                00219541
                April 2017
                April 21 2017
                : 232
                : 4
                : 806-817
                Article
                10.1002/jcp.25485
                c3463108-d770-4057-b9d3-0c6d5de8cfb7
                © 2017

                http://doi.wiley.com/10.1002/tdm_license_1

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